1. Field of the Invention
The present invention relates to an auxiliary air flow control system suitable for use in internal combustion engines, and more specifically to a system equipped with an auxiliary air flow constriction structure which is capable of optimally constricting a controlled air flow produced by an auxiliary air flow control valve, depending on changes in the engine temperature.
2. Description of the Prior Art
In recent years, to suit different operating conditions of the engine, there have been proposed and developed various auxiliary air flow control systems for internal combustion engines. The typical auxiliary air flow control system has at least an electronically-controlled auxiliary air flow control valve whose opening degree is electronically controlled in response to a control signal from a control unit which unit usually employs an input interface connected to sensors for monitoring operating conditions of the engine such as engine load, engine speed and the like, and a cooling-water temperature dependent auxiliary air flow constricting valve whose opening degree is controlled depending on the cooling-water temperature (called engine coolant temperature). For this purpose, the latter valve (the cooling-water temperature dependent auxiliary air flow constricting valve) has a temperature-sensing portion for sensing a temperature of cooling water which circulates in a water jacket in the engine. The cooling water temperature is usually regarded as an engine temperature. Ordinarily, the auxiliary air flow control valve and the auxiliary air flow constricting valve are fluidly disposed in series to each other in an auxiliary air passage arranged parallel to the intake-air passage passing through the throttle valve, so that the auxiliary air flow control valve is arranged upstream of the auxiliary air flow constricting valve. The auxiliary air passage is connected to the intake-air passage in such a manner as to bypass the throttle valve for the purpose of diverting intake air from the upstream of the throttle valve to the downstream of the throttle valve therethrough. A flow rate of auxiliary air, passing through the auxiliary air flow control valve, is properly adjusted in dependent on the engine operating conditions, and then the properly controlled auxiliary air flow is constricted by means of the auxiliary air flow constricting valve depending on the engine temperature. The previously-noted constriction or adjustment of auxiliary air flow is effective to maintain the engine speed at a predetermined engine speed during idling. One such conventional auxiliary air flow control system has been disclosed in Japanese Utility Model Provisional Publication No. 62-119446. The auxiliary air flow constricting valve employed in the prior art system as described in the Japanese Utility Model Provisional Publication No. 62-119446 has a typical characteristic of coolant-temperature versus auxiliary air-flow constriction, in which characteristic a constricting amount of auxiliary air flow is gradually increased in accordance with an increase in coolant temperature sensed by a temperature-sensing portion incorporated into the auxiliary air flow constricting valve, and held constant after the sensed coolant temperature reaches and exceeds a predetermined temperature, i.e., after warm up of the engine. In other words, according to the above-mentioned typical coolant-temperature versus auxiliary air-flow constriction characteristic, the total flow rate of auxiliary air flowing through the auxiliary air passage (or the by-pass passage) is gradually decreased (essentially in a linear fashion) during cold-engine operation and then held constant after warm-up, as indicated by one-dotted line of FIG. 2. The prior art auxiliary air flow control system suffers from the following drawback. That is, in the event that the engine temperature (the coolant-temperature) rises and exceeds the predetermined water temperature, the temperature of intake air rises and thus the air density tends to be lowered. In this case, with respect to engine load actually applied, a required air flow rate must be increased owing to the lowering of air density. However, in the case of the conventional auxiliary air flow constricting valve having the previously-noted coolant-temperature versus auxiliary air-flow constriction characteristic, the constricting amount of auxiliary air flow is held constant within a high-temperature range above the predetermined temperature. This could result in lack of auxiliary air flow. Thus, there is a possibility that the prior art system does not satisfactorily maintain a target idle speed owing to the lack of auxiliary air flow at high coolant temperatures above the predetermined temperature. During operation of the prior art system within the high-temperature range, there is a tendency that the engine speed tends to drop in comparison with the target idle speed during idling.